Several methods are known for manufacturing composite parts with hollow bodies, in particular the RTM/LRI (Resin Transfer Molding/Liquid Resin Infusion) methods and the prepreg method. Also known are ply draping or filament winding techniques that can be applied to either of the cited methods.
In an RTM/LRI method, an assembly of fibrous elements is positioned in a particular manner around a supporting element. This fibrous element assembly forms a preform that, with its supporting element, is then placed in a mold inside which a resin is injected, generally under a vacuum or pressure. The resin is then polymerized, thereby forming a rigid and light composite material made up of fibers and polymerized resin.
The RTM/LRI method has great flexibility and allows the production of pieces with a complex geometry. Indeed, the fibers of the fibrous elements initially being dry, they can be put into place more easily to assume the shape of the supporting element.
The RTM/LRI method also has the advantage of requiring fewer assembly operations, since this possibility of producing pieces with complex shapes makes it possible to avoid making several pieces with less complex shapes to then assemble them together.
However, the fibrous elements initially being dry, their fibers are also more easily susceptible to changing orientation or having alignment defects before casting and polymerization of the resin.
Given that in such a composite material the alignment of the fibers defines and determines the stress transmission lines, in particular in compression and buckling, it is understood that the presence of alignment defects of the fibers in the final pieces has negative repercussions on the mechanical properties for picking up and transmitting stresses of said piece.
This defect can be improved by using a preform including a very small resin portion that serves as fixer in order to strengthen the connection of the fibers to each other, this method requiring, other than the operation of adding the fixer, a stoving operation of the formed preform to activate the fixer.
In a prepreg method, the fibrous elements are no longer dry, but preimpregnated with at least partially polymerized resin.
The fact that the fibers of these fibrous elements are already impregnated with resin greatly limits their mobility and change of orientation.
However, the prepreg method has drawbacks and in particular does not allow, or allows with difficulty, the production of pieces with complex geometry, the resin greatly limiting the shaping of the fibrous elements. It is therefore also difficult to obtain a good compression rate of the fibers between the supporting element and the mold during the final polymerization.
Moreover, such a method multiplies the manufacturing steps, since the fibrous elements must undergo a preimpregnation before draping and possibly a partial pre-polymerization before shaping and final polymerization.
There is therefore a significant interest in improving the RTM/LRI methods and providing a method making it possible to keep the advantages thereof while also ensuring proper orientation of the fibers, good compaction thereof in significant numbers.
Several prior art documents have already tried to resolve this problem.
Document WO 2007/060306 describes a method for producing a rod in a composite material from a single dry preform shaped before casting of the resin. In this method, the preform has opposite edges each having a complementary beveled surface created during its winding around the supporting element, said edges being superimposed before being joined by the resin.
However, in such a method, there is still a significant risk of movement of the fibers before molding. This method also has drawbacks related to mass optimization to ensure the alignment of the longitudinal fibers.
Document WO 2007/031649 describes a method for manufacturing a composite piece from a plurality of dry and preimpregnated preforms. The correct alignment of the fibers is ensured locally by this use of preimpregnated preforms.
However, this method still requires several polymerization steps.
The filament winding techniques, aside from having the same difficulties as mentioned above with the RTM or prepreg methods, can have the constraint of fiber continuity to perform the winding and therefore a handicap for optimizing the mass of the obtained structure.